HSC Exam Practice

Sample response. An Arrhenius acid is a substance that produces H⁺ (hydrogen) ions when dissolved in water. An Arrhenius base is a substance that produces OH⁻ (hydroxide) ions when dissolved in water.

Marking notes. 1 mark for Arrhenius acid definition (must reference H⁺ production in aqueous solution). 1 mark for Arrhenius base definition (must reference OH⁻ production in aqueous solution).

Sample response. Brønsted-Lowry acid: CH₃COOH — it donates a proton (H⁺) to water. Brønsted-Lowry base: H₂O — it accepts the proton (H⁺) from CH₃COOH, forming H₃O⁺.

Marking notes. 1 mark for identifying CH₃COOH as the acid (must state “donates H⁺” or equivalent). 1 mark for identifying H₂O as the base (must state “accepts H⁺” or equivalent). Merely naming the species without explaining the proton transfer earns 0.

Sample response. Conjugate pair 1: CH₃COOH (left, acid) and CH₃COO⁻ (right, conjugate base) — they differ by exactly one H⁺; CH₃COOH has lost H⁺ to become CH₃COO⁻. Conjugate pair 2: H₂O (left, base) and H₃O⁺ (right, conjugate acid) — they differ by exactly one H⁺; H₂O has gained H⁺ to become H₃O⁺.

Marking notes. 1 mark per correctly named pair (both species named, one from each side) = 2 marks. 1 mark per correct explanation of the H⁺ difference for each pair = 2 marks. Common error: listing species from the same side as a conjugate pair — 0 marks for that pair.

Sample response. NH₃ is a Brønsted-Lowry base because it accepts a proton (H⁺) from water: NH₃(aq) + H₂O(l) ⇌ NH₄⁺(aq) + OH⁻(aq). The OH⁻ produced in solution comes from the water molecule that donated H⁺ to NH₃, not from NH₃ itself. NH₃ cannot be classified as an Arrhenius base because the Arrhenius model requires a base to produce OH⁻ in aqueous solution, and NH₃ does not contain any OH⁻ in its molecular structure (formula: NH₃ — one N, three H, no O).

Marking notes. 1 mark — correct Brønsted-Lowry equation with equilibrium arrow (⇌). 1 mark — NH₃ identified as the proton acceptor / base. 1 mark — explanation that OH⁻ comes from water, not from NH₃. 1 mark — Arrhenius cannot classify NH₃ because it contains no OH⁻ (must reference the definition). Do not accept “NH₃ releases OH⁻” — that is the target misconception.

Sample response. H⁺(aq) and H₃O⁺(aq) represent the same chemical species: both refer to the proton (hydrogen ion) in aqueous solution. H⁺ is shorthand; in reality a free proton cannot exist in water as it immediately bonds to a water molecule’s lone pair to form the hydronium ion, H₃O⁺. H₃O⁺ is preferred (and required) when explaining acid-base behaviour at the molecular level using the Brønsted-Lowry model, as it explicitly shows water acting as the proton acceptor (base). H⁺ is acceptable in calculations (e.g. pH = −log[H⁺]) where the distinction is not being tested.

Marking notes. 1 mark — both represent the same species / H⁺ is shorthand for H₃O⁺. 1 mark — H₃O⁺ is preferred/required in Brønsted-Lowry/molecular-level explanations because it shows water as the proton acceptor. 1 mark — H⁺ is acceptable in pH calculations / Arrhenius context.

Sample response. The main limitation of the Arrhenius model is that it is restricted to aqueous solutions: it defines acids and bases only by the ions they produce in water. As a result, it cannot describe acid-base reactions that occur in non-aqueous media, and it cannot classify bases that do not contain OH⁻ (such as NH₃, HCO₃⁻, and CO₃²⁻). For example, the reaction HCl(g) + NH₃(g) → NH₄Cl(s) is a complete acid-base reaction producing a salt in the gas phase with no water present, and the Arrhenius model cannot describe or classify either reactant in this reaction.

Marking notes. 1 mark — limitation correctly stated as “restricted to aqueous solutions” or “cannot classify bases without OH⁻”. 1 mark — names a specific reaction the Arrhenius model cannot describe (HCl(g) + NH₃(g) → NH₄Cl(s) or NH₃ dissolving to form a basic solution). 1 mark — explains why the named reaction cannot be described by Arrhenius (no aqueous medium / NH₃ has no OH⁻). Award 0 for “Arrhenius was wrong” as a standalone statement without qualification.

Part (a) sample response. Using the Arrhenius model only, NH₃ cannot be classified as a base (it does not contain OH⁻ and the table shows it cannot donate H⁺ and does not contain OH⁻ in its structure, yet pH = 11.1 indicates it is basic — Arrhenius cannot explain this). HCO₃⁻ also cannot be classified as a base by Arrhenius for the same reason (no OH⁻ in structure). H₂O presents difficulty too: Arrhenius does not classify water as either an acid or a base under normal circumstances. Accept: NH₃, HCO₃⁻, and H₂O as substances that cannot be classified by Arrhenius as either acid or base (the table data shows they contain no OH⁻ and either do not appear to produce H⁺ in the typical Arrhenius sense, or produce basic solutions Arrhenius cannot explain).

Marking notes (a). 1 mark for correctly identifying NH₃ with justification (no OH⁻, Arrhenius cannot classify it as a base despite pH > 7). 1 mark for any other valid substance (HCO₃⁻ or H₂O) with justification. Accept one correct answer with clear justification for 2 marks.

Part (b) sample response. The table shows NH₃ has pH = 11.1, confirming it is basic in aqueous solution, yet it contains no OH⁻ in its structure and cannot donate H⁺. The Arrhenius model cannot classify NH₃ as a base for these reasons. The Brønsted-Lowry model classifies NH₃ as a base because it can accept a proton from water: NH₃(aq) + H₂O(l) ⇌ NH₄⁺(aq) + OH⁻(aq). NH₃ is the proton acceptor (base) and H₂O is the proton donor (acid). The OH⁻ in solution comes from water losing H⁺ to NH₃, not from NH₃ itself. Brønsted-Lowry explains this because it defines a base by proton acceptance, not by OH⁻ production.

Marking notes (b). 1 mark — references the data (pH 11.1 / no OH⁻ in structure) to show why Arrhenius fails. 1 mark — correct Brønsted-Lowry equation with equilibrium arrow. 1 mark — identifies NH₃ as proton acceptor (base) and H₂O as proton donor (acid) in the equation. 1 mark — explains OH⁻ comes from water, not from NH₃. 1 mark — states the key distinction: Brønsted-Lowry defines basicity by proton acceptance, not OH⁻ production.

Part (c) sample response. Both H₂O and HCO₃⁻ are described as amphiprotic substances — they can act as either a proton donor or a proton acceptor depending on their reaction partner. HCO₃⁻ acting as an acid (donating H⁺): HCO₃⁻(aq) + H₂O(l) ⇌ H₃O⁺(aq) + CO₃²⁻(aq).

Marking notes (c). 1 mark — correct term: amphiprotic. 1 mark — correct equation showing HCO₃⁻ donating H⁺ (with equilibrium arrow; product must be CO₃²⁻). Accept: HCO₃⁻ + OH⁻ → CO₃²⁻ + H₂O as an alternative (HCO₃⁻ donating to a base).

Sample response. The claim is an oversimplification and is substantially incorrect on both counts: Arrhenius was not made redundant, and Arrhenius was not proved incorrect.

The Arrhenius model (1884) correctly describes all strong acid and strong base reactions in aqueous solution and remains the basis for standard pH calculations. For example, HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l): Arrhenius correctly predicts HCl provides H⁺ and NaOH provides OH⁻ in water, and the model is still applied routinely today in calculating pH. A model that correctly predicts a large class of observations is not “incorrect” or “redundant” — it is limited in scope. The correct language is that the Arrhenius model is “limited to aqueous systems” or “cannot classify bases without OH⁻.”

However, the Brønsted-Lowry model (1923) does extend beyond Arrhenius in two critical ways. First, it explains NH₃ as a base: NH₃(aq) + H₂O(l) ⇌ NH₄⁺(aq) + OH⁻(aq). Arrhenius cannot classify NH₃ as a base because NH₃ contains no OH⁻, yet the solution is clearly basic (pH ~11). Brønsted-Lowry resolves this by defining a base as a proton acceptor — NH₃ accepts H⁺ from water via its nitrogen lone pair, and the resulting OH⁻ comes from water, not from NH₃. Second, Brønsted-Lowry accounts for non-aqueous acid-base reactions: HCl(g) + NH₃(g) → NH₄Cl(s) is a proton transfer in the gas phase with no water present — an observation Arrhenius cannot address at all.

Neither model is unlimited. Brønsted-Lowry also has a limitation: it cannot describe Lewis acid-base reactions (e.g. BF₃ + F⁻ → BF₄⁻) that involve no proton transfer. The claim therefore fails on two grounds: Arrhenius was not made redundant (it remains valid and used for aqueous strong acid-base chemistry) and was not proved incorrect (it accurately predicts a well-defined class of reactions). Brønsted-Lowry is a more general framework that extends Arrhenius rather than replaces it.

Marking criteria:

• 1 mark — identifies that Arrhenius remains valid/used for aqueous strong acid-base chemistry (e.g. HCl + NaOH), with an example
• 1 mark — correct distinction between “limited” and “incorrect”: Arrhenius is limited in scope, not disproved
• 1 mark — uses NH₃ + H₂O ⇌ NH₄⁺ + OH⁻ to show where models diverge; identifies Arrhenius failure (no OH⁻ in NH₃) and Brønsted-Lowry success (proton acceptance)
• 1 mark — uses a non-aqueous reaction (HCl(g) + NH₃(g) → NH₄Cl(s) or equivalent) to show Arrhenius cannot describe gas-phase reactions; Brønsted-Lowry can
• 1 mark — identifies a limitation of the Brønsted-Lowry model itself (Lewis acid-base reactions without proton transfer)
• 1 mark — reaches a clear evaluative judgement that refutes the claim using both points above: Arrhenius is not redundant (still used, still valid for its scope) and is not incorrect (accurately predicts its class of reactions); Brønsted-Lowry extends rather than replaces it